How accurate are ultrasound

Pamela • Blessing with a 2 little boys and baby boy due april 23rd 2019
I'm wondering how accurate the weight is of the baby when they check on the ultrasound? I had an ultrasound done at 34 weeks and 4 days I am now 35 weeks and the ultrasound guy told me that the baby weighted 5lb and 13oz he said that's going to be a big baby! How accurate is this? I'm hoping to to get my membrane sweep done at 38 weeks